Magnetism and Correlated Electrons in LaCr$_2$Ge$_2$N

TL;DR

This study synthesizes and characterizes LaCr2Ge2N, a new Cr-based layered nitride in the CeCr2Si2C-type family, to understand how electron counting governs magnetism and electron correlations. Through structural, magnetic, thermodynamic, transport, and high-pressure measurements complemented by first-principles calculations, the work reveals short-range antiferromagnetic correlations near 460 K and long-range antiferromagnetic order at 14 K, alongside a possible electronic ordering near 378 K and a significantly enhanced Sommerfeld coefficient (γ ≈ 51–53 mJ/mol-f.u./K^2) relative to band values. Density functional theory predicts a striped antiferromagnetic ground state driven by Cr-3d bands that are nearly flat near EF, with EF displaced by about 0.2 eV toward these flat bands due to the electron count, explaining the emergence of magnetism in LaCr2Ge2N compared to the paramagnetic LnCr2Si2C series. The work demonstrates how electron filling in this structural family tunes magnetic ground states and correlations, establishing LaCr2Ge2N as a platform for studying frustrated magnetism in layered Cr-based materials.

Abstract

We report the synthesis, structure and physical properties of a new quaternary nitride LaCr$_2$Ge$_2$N. The compound crystallizes in the CeCr$_2$Si$_2$C-type structure (P4/mmm), featuring distinctive Cr$_2$N square sheets within Cr$_2$Ge$_2$N block layers. Physical characterizations reveal enhanced electron correlations evidenced by a Sommerfeld coefficient substantially larger than band calculations and pressure-induced deviation from Fermi-liquid behavior. Magnetic measurements show short-range antiferromagnetic correlations developing around 460 K, followed by long-range magnetic ordering at 14 K. Additionally, subtle anomalies at 378 K suggest possible electronic ordering. First-principles calculations reveal nearly-flat Cr-3d bands near the Fermi level and predict a striped antiferromagnetic ground state. This work demonstrates how electron count variation in the CeCr$_2$Si$_2$C-type structure family leads to magnetic ordering in LaCr$_2$Ge$_2$N, contrasting with the paramagnetic behavior of LnCr$_2$Si$_2$C compounds.

Figures (5)

• Figure 1: (a) The X-ray powder diffraction pattern of LaCr$_2$Ge$_2$N polycrystalline sample. (b) Crystal structure of LaCr$_2$Ge$_2$N. (c) Schematic of the CrGe$_4$N$_2$ octahedron.
• Figure 2: Temperature dependence of magnetic susceptibility $M/H$ for LaCr$_2$Ge$_2$N measured at $\mu_0H = 4$ T. The upper inset shows the low-temperature region highlighting the anomaly at $\sim$14 K. The lower inset displays the full temperature range up to 800 K, revealing a broad maximum around 460 K characteristic of short-range antiferromagnetic correlations.
• Figure 3: Low-temperature specific heat of LaCr$_2$Ge$_2$N plotted as $C/T$ versus $T^2$. The red solid line represents the fit using $C(T) = \gamma T + C_{\rm Debye}(T)$ in the range 18-40 K. The inset shows the temperature dependence of estimated magnetic entropy change obtained by integrating $C_{\rm mag}/T$.
• Figure 4: Temperature dependence of electrical resistivity for LaCr$_2$Ge$_2$N. (a) Resistivity measured under various pressures up to 13 GPa, with black solid lines showing power-law fits ($\rho = \rho_0 + AT^n$) at low temperatures. (b) Pressure dependence of the power-law exponent $n$. (c) High-temperature resistivity at ambient pressure, revealing an anomaly near 378 K that coincides with the susceptibility feature.
• Figure 5: (a) Orbital-projected band structure of LaCr$_2$Ge$_2$N along high-symmetry paths. (b) The corresponding total and orbital-projected density of states.